An EU report found X has the highest proportion of Russian disinformation of any major social network.
A senior official warned that the site "has to comply with hard law" over disinformation.
X said that it disagreed with the "overall framing" of the data published in the EU's report.
The EU has warned that X, the social media company owned by Elon Musk, must clamp down on disinformation, after a study found that Russian propaganda was proliferating on the site.
A report from the European Commission found that X, formerly Twitter, had the highest proportion of Russian disinformation of any of the major social media platforms, as Musk continues to overhaul the social network's anti-disinformation policies.
The report examined 6,000 posts from Spain, Slovakia, and Poland made across a number of social media platforms.
Speaking to reporters on Tuesday, in comments reported by Bloomberg, European Commission Vice President Vera Jourova described X as "the platform with the largest ratio of misinformation or disinformation posts."
"This is a multi-million euro weapon of mass manipulation aimed both internally at the Russians as well as Europeans and the rest of the world," she continued, as cited by the BBC.
"As Twitter has been designated as a very large online platform, of course there are obligations given by hard law. So, my message to Twitter is you have to comply with hard law and we are watching what you are doing," she added, per Bloomberg.
The EU said in its report that it began its research before Twitter rebranded to X, and therefore decided to refer to it as Twitter in the report.
Musk has faced criticism over the spread of Russian propaganda on X, with a separate report from the European Commission finding that changes made to the site's safety policies have helped spread false information about the war in Ukraine.
In May, he pulled X, then Twitter, out of the EU's disinformation code of practice, a voluntary agreement signed by Google, Meta, Microsoft, and TikTok.
The EU has put pressure on global tech companies to clamp down on disinformation and fake news. Under the Digital Services Act, which came into effect last month, social platforms face fines of up to 6% of their annual revenue if they fail to do so.
"Mr Musk knows that he is not off the hook by leaving the code of practice, because now we have the Digital Services Act fully enforced," Jourova said, per the BBC.
Posting on X, the company's Global Government Affairs team said that X was committed to complying with the Digital Services Act.
"We disagree with the overall framing of this data and believe that the data does not fit the narrative being covered in the media," they said.
"This important debate should take into account the full range of actions taken by platforms & recognize the importance of protecting free expression."
X did not immediately respond to a request for comment from Insider, made outside normal working hours.
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